The present invention relates to the formation of polymer/zeolite nanocomposite membranes, and in particular to using such membranes for fuel cell applications.
Proton exchange membrane fuel cell (PEMFC) systems have been determined to be approximately two to three times as efficient as conventional internal combustion engine (ICE) power systems. The power density, volume and weight of PEMFC systems approach that of an ICE power system, making PEMFC systems ideal for transportation applications. Prototype PEMFC vehicles have shown that hydrogen fed, PEMFC powered vehicles are capable of performance comparable with ICE vehicles in all respects, however the cost of a mass produced PEMFC system is currently 10 times greater than a comparable ICE power system, and no national hydrogen refueling network exists. Fuel cell cost is expected to come down as the technology is developed further, but the costs associated with developing a hydrogen distribution network would be tremendous.
However, there would be no need for a new national distribution network, if either reformed hydrogen or methanol fed PEMFC powered vehicles were used, because a liquid hydrocarbon (gasoline) distribution network is already in place. PEMFCs can be run on hydrogen gas generated by reforming a liquid hydrocarbon fuel such as methanol, ethanol or gasoline, as long as carbon monoxide levels in the hydrogen feed stream are reduced to less than 10 ppm. Use of feed hydrogen with more than 10 ppm CO causes anode catalyst poisoning and fuel cell performance suffers. Unfortunately, system bulk and complexity required to reduce CO levels in the hydrogen feed gas to 10 ppm during steady operation and 100 ppm during transient operation makes it impractical for use in a vehicle. Interestingly, use of both reformed hydrogen fed PEMFC power plants could be realized if the temperature operation range of their proton exchange membrane material was extended from 80° C. up to 140° C. CO tolerance of the PEMFC anode catalyst has been shown to increase greatly with temperature, and would eliminate the need for bulky CO removal equipment for the reformed hydrogen feed stream. In addition, higher temperature operation can also facilitate thermal management/heat utilization of the fuel cell stack, and potentially simplify fuel cell water management. However, current proton exchange membranes are not suitable for operation at these temperatures due to dehydration problems and mechanical instability. One of the major technology breakthroughs required for PEMFCs is a membrane material that operates at higher temperatures.
Direct methanol fuel cells (DMFCs), which take liquid methanol instead of H2 fuel, enjoy a competitive advantage over H2-air fuel cell system in terms of the refueling infrastructure, storage/transport system and safety concerns. DMFC is especially promising for powering portable electronics such as cell phones and laptop computers. The crossover of methanol through the polymer electrolyte membrane and the low activity of the reported Pt based catalyst for methanol oxidation limit performance of DMFCs and restrict their potential applications. In terms of methanol crossover, besides the loss of fuel, another disadvantage is that it will lead to a significant performance loss due to the formation of “mixed potentials”, as a result of concurring of oxygen reduction and methanol oxidation on the Pt cathode electrocatalysts.
Experimental results show that methanol crossover through Nafion membrane leads to a significant performance loss in DMFC and this phenomenon is more severe with the increase of methanol feed concentration. Therefore, there exists a need to develop a composite membrane that is able to reduce the methanol crossover while still maintaining its high proton conductivity; a membrane that is able to combine the high proton conductivity of a polymer with the hydration and mechanical stability of an inorganic material in order to get a membrane capable of elevated (e.g., 140° C.) operation.
One promising strategy for efficient proton conduction and methanol blocking at elevated temperatures is to incorporate inorganic nanoparticles inside the membrane. Previous attempts were based on the idea of combining the high proton conductivity of Nafion with the thermally stable, hydrophilic material, such as silica, in order to maintain hydration at high temperatures and to physically block methanol crossover. However, introducing silica dilutes the sulfonic acid number concentration, resulting in a loss in membrane proton conductivity. An ideal composite membrane additive would be able to increase the proton concentration, while still improving hydration/mechanical properties and blocking methanol crossover at high temperatures.